1. Technical Field
This disclosure relates to the preparation of specimens inside focused ion-beam (FIB) microscopes and the preparation of specimens for later analysis in the transmission electron microscope (TEM), and apparatus to facilitate these activities.
2. Background
The use of focused ion-beam (FIB) microscopes has become common for the preparation of specimens for later analysis in the transmission electron microscope (TEM). The structural artifacts, and even some structural layers, in the device region and interconnect stack of current integrated-circuit devices can be too small to be reliably detected with the secondary electron imaging in a Scanning Electron Microscope (SEM), or FIB, which offers a bulk surface imaging resolution of approximately 3 nm. In comparison, TEM inspection offers much finer image resolution (<0.1 nm), but requires electron-transparent (<100 nm thick) sections of the sample mounted on 3 mm diameter grid disks. The recent advances in TEM microscopy, including the atomic resolution transmission electron microscopy, require even thinner specimens, of the order of tenths of a nanometer thick. A specimen having such thickness is very fragile and can be easily perforated during milling. It is very important to be able to monitor a milling progress and identify a moment when the milling should stop, so the resulting TEM specimen is thin enough for the quality TEM investigation and not destroyed at the same time.
For in-situ lift-out to be practical for large-wafer FIB chambers or for high-volume TEM specimen preparation, the number of specimens should be examined without the need to vent the vacuum chamber. Venting of the FIB vacuum chamber is time consuming. It disables the FIB during the venting and re-pumping cycle, and can eventually degrade the performance of the FIB if repeated often.
The in-situ lift-out technique is a series of FIB milling and specimen-translation steps used to produce a site-specific specimen for later observation in a TEM or other analytical instrument. Details on methods of in-situ lift-out may be found in the specifications of U.S. Pat. Nos. 6,420,722 and 6,570,170. These patent specifications are incorporated into this application by reference, but are not admitted to be prior art with respect to the present application by their mention in the background.
The process of in-situ lift-out can be simplified into three successive steps. The first is the excision of the lift-out sample from which the TEM specimen will be fabricated using focused ion-beam milling and extraction of the lift-out sample from its trench. The second is the holder-attach step, during which the lift-out specimen is translated on the probe tip point to the TEM specimen holder, typically a TEM grid. Then it is attached to the TEM holder (typically with ion beam-induced metal deposition) and later detached from the probe tip point. The third and final step is the thinning of the lift-out specimen into an electron-transparent thin section (the “specimen” here) using focused ion beam milling.
There is a need for faster and more accurate method and apparatus for monitoring the milling progress of a specimen. It would be advantageous if the TEM grid with the TEM specimen attached to it could be placed in a variable tilt TEM grid holder, thus allowing gradual adjustment of the TEM specimen orientation in the limited space of a DB-FIB apparatus, without venting the DB-FIB chamber, and hence more accurate monitoring of milling operation.